The field of the present invention is fine mesh screening systems including the use of vibration to assist screening.
Traditional vibratory screening systems typically include a base, a frame resiliently mounted to the base with a screen or screens extending across the frame. A low frequency vibratory drive in the speed range of 8 Hz to 30 Hz with eccentric weights is mounted to the frame. Specific vibratory motions are established in the frame by the low frequency vibratory drive depending upon the phase of the eccentric weights, generating screen accelerations up to the 7 g range. One such vibratory screen separator is illustrated in U.S. Pat. No. 5,456,365, the disclosure of which is incorporated herein by reference.
The foregoing devices have been used for screening a wide variety of materials in size and shape. Further, such devices handle a variety of flow conditions for material to be screened from dry to fully entrained in liquid.
A number of circumstances and conditions can reduce screening efficiency with such devices. For example, screens can be blinded by certain materials which are not dislodged by the vibratory action. Another problem can be that finer materials float above the low frequency vibrating screen.
In an effort to overcome certain of the deficiencies of low frequency vibration, ultrasonic vibrators have been employed in conjunction with low frequency vibratory drives. Ultrasonic vibrators have been mounted to separator frames with a direct mechanical attachment to the screens at the centers thereof. Reference is made to U.S. Pat. No. 5,653,346. Alternatively, ultrasonic drives have been supported directly by the screen. Reference is made to U.S. Pat. No. 5,143,222. Additionally, ultrasonic vibrators have been mounted to the peripheral frame of the screen. Reference is made to U.S. Pat. No. 5,398,816, the disclosure of which is incorporated herein by reference.